A frame in 802.11b wireless LAN system is composed of a preamble sequence, a frame header sequence and a data sequence. As data are transmitted in a burst manner in a wireless LAN, a receiver must complete all synchronization processes during the period of receiving the preamble sequence, and carrier frequency estimation is the most important step therein. According to the provision of the wireless LAN standard that crystal oscillator stability shall be controlled within the range of ±25 ppm, the maximum permissible carrier frequency offset for a receiver can be calculated as to be ±125 KHz.
802.11b wireless LANs are required to be able to normally operate in extreme environments, for example, it is required to achieve stable and reliable transmission at a signal-to-noise ratio (SNR) of less than 0 dB under the 1 Mbps mode. Thus, carrier frequency estimation is required to ensure both a high accuracy at an SNR of less than 0 dB and a sufficient estimation range.
In a general case, a 802.11b wireless LAN adopts a delayed correlation method to directly obtain rough estimates of carrier frequency offset. Performance of this method depends on the length of delay. Specifically, a small delay results in a wide estimation range and a poor estimation accuracy; and conversely, a greater delay leads to a higher accuracy and a narrower estimation range though due to phase ambiguity issues.